Electrographic imaging and development processes, e.g., electrophotographic imaging processes and techniques, have been extensively described in both the patent and other literature, for example, U.S. Pat. No. 2,221,776, issued Nov. 19, 1940; U.S. Pat. No. 2,277,013, issued Mar. 17, 1942; U.S. Pat. No. 2,297,691, issued Oct. 6, 1942; U.S. Pat. No. 2,357,809, issued Sept. 12, 1944; U.S. Pat. No. 2,551,582, issued May 8, 1951; U.S. Pat. No.2,825,814, issued Mar. 4, 1958; U.S. Pat. No. 2,833,648, issued May 6, 1958; U.S. Pat. No. 3,220,324, issued Nov. 30, 1965; U.S. Pat. No. 3,220,831, issued Nov. 30, 1965; U.S. Pat. No. 3,220,833, issued Nov. 30, 1965, and many others. Generally, these processes have in common the steps of forming a latent electrostatic charge image on an insulating electrographic element, such as a photoconductive insulating layer coated on a conductive support. The electrostatic latent image is then rendered visible by a development step in which the charge image-bearing surface of the electrographic element is brought into contact with a suitable developer composition.
Many types of developer compositions, including both dry developer compositions and liquid developer compositions, have been proposed heretofore for use in the development of latent electrostatic charge images. Dry developer compositions typically suffer from the disadvantage that distribution of the dry toner powder contained therein on the surface of the electrographic element bearing the electrostatic latent image is difficult to control. These dry developers have the further disadvantage that the use thereof may create excessive amounts of dust and that high resolution is often difficult to obtain due to the generally relatively large size of the dry developer powder particles.
Many of the disadvantages accompanying the use of dry developer compositions have been avoided in the past by the use of a liquid developer of the type described, for example, in Metcalfe et al, U.S. Pat. No. 2,907,674 issued Aug. 6, 1959. Such developers usually comprise an electrically insulating liquid which serves as a carrier and which contains a stable dispersion of charged particles known as toner particles comprising a pigment such as carbon black, generally associated with a resinous binder, such as, for example, an alkyd resin. A charge control agent is often included to stabilize the magnitude and polarity of the charge on the toner particles. In some cases, the binder itself serves as a charge control agent.
To achieve suitable physical stability of the toner particles dispersed in conventional liquid electrographic developers, any of several types of various "stabilization" additives are incorporated in such a liquid developer to prevent the toner particles from settling out of the carrier liquid. Typical of such additives are those described in York, U.S. Pat. No. 2,899,335 issued Aug. 11, 1955 and various types of dispersants as described, for example, in British Pat. No. 1,065,796.
One problem which has continued to persist with conventional "stabilized" liquid electrographic developer compositions as described above is that even these "stabilized" compositions, which contain various kinds of stabilization additives, tend to become "deactivated" within a few weeks and the toner particles tend to agglomerate or settle out of the developer. As a consequence, the resultant liquid developer composition containing conventional liquid developer toner particles tends to become incapable of producing electrostatic prints of good quality and density. (See British Pat. No. 1,065,796 noted above). This deactivation of conventional stabilized liquid developers is particularly troublesome because once the toner particles settle out of the developer suspension, it is often difficult to redisperse them. And, even if redispersed, it is often found that the redispersed liquid developer does not possess the same developer characteristics as the original developer.
Developer stability, even in conventional so-called "stabilized" liquid developers has been and is still a difficult problem to overcome. This is one reason, in addition to convenience, that liquid developers are often prepared in the form of so-called"concentrates" i.e., mixtures of resins, pigments and/or dyes with a low liquid content. These concentrates are stable and exhibit a relatively long shelf life. The loss of "stability" which occurs in conventional liquid electrographic developers, as noted hereinabove, occurs primarily in the diluted form of the concentrate which is the "working" form of the developer, i.e., the form of developer composition actually used in most electrographic developing processes.
Stability in "working" liquid developer compositions may be improved to some extent, as noted above in the afore-mentioned York patent, by the use of various stabilization agents. These additives are most effective in a developer when used in conjunction with toner particles having a very small particle size. However, even in these situations where stability is achieved in working developers, this stability is often accompanied by too high a developer sensitivity which tends to result in a high degree of background density in the resultant liquid developed electrographic images.
A further problem associated with conventional "stabilized" liquid developers has been the problem of replenishment. That is, once the developer is used to produce a number of developed images, the developer becomes depleted of toner particles and must be replenished. However, as might be expected in any dispersed system wherein a delicate equilibrium must necessarily be maintained to avoid settling out or precipitation of the dispersed particles, replenishment by injecting new particles into the suspension is a particularly complex problem.
In addition to the "stabilized" liquid developers described above, various "redispersible" liquid developers have been formulated. These "redispersible" developers are characterized by toner particles which, upon settling out of suspension with the liquid carrier vehicle of the developer, are readily redispersed in the liquid carrier and, when so redispersed, exhibit developer characteristics similar to the original developer. However, various problems still exist with many of these "redispersible" developers. For example, the toner particles of many of these developers do not adhere well, i.e., they cannot readily be fixed, except to rough-surfaced toner-image receiving sheets such as conventional zinc oxide coated papers, using preferred fixing temperatures of about 100.degree. C. or less. These developers, therefore, cannot be employed, except with further binder addenda, in various transfer processes such as many of the so-called TESI processes (as described in "Electrophotography" by R. M. Schaffert, p. 67-p. 85, The Focal Press, 1965), because these processes use smooth-surfaced toner-image receiving elements, such as dielectric resin-coated papers, i.e., papers coated with a film-forming dielectric resin. Still other available redispersible developers, although redispersible at ordinary room temperatures, exhibit pronounced caking or agglomeration of the toner particles when subjected to extended periods of storage (e.g., 24 hours) at temperatures above room temperature, i.e., at about 55.degree.-60.degree. C., and cannot be readily redispersed.